Highlight ImageButton inside GridView or DataList when clicked in ASP.Net

You can use Border Style to highlight the button

 HTML

<div>
    <asp:DataList runat="server" ID="dlContacts">
        <HeaderTemplate>
            <table border="0" cellpadding="2" cellspacing="2">
                <tr>
                    <th>
                        Id
                    </th>
                    <th>
                        Name
                    </th>
                    <th>
                        Country
                    </th>
                    <th>                            
                    </th>
                </tr>
        </HeaderTemplate>
        <ItemTemplate>
            <tr>
                <td>
                    <%# Eval("Id") %>
                </td>
                <td>
                    <%# Eval("Name") %>
                </td>
                <td>
                    <%# Eval("Country") %>
                </td>
                <td>
                    <asp:ImageButton ID="ImageButton1" runat="server" ImageUrl="~/Images/transfer.png"
                        OnClick="ImageButton1_Click" />
                </td>
            </tr>
        </ItemTemplate>
        <FooterTemplate>
            </table>
        </FooterTemplate>
    </asp:DataList>
</div>

Namespace

using System.Data;

 C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        DataTable dt = new DataTable();
        dt.Columns.AddRange(new DataColumn[3] { new DataColumn("Id", typeof(int)),
                        new DataColumn("Name", typeof(string)),
                        new DataColumn("Country",typeof(string)) });
        dt.Rows.Add(1, "John Hammond", "United States");
        dt.Rows.Add(2, "Mudassar Khan", "India");
        dt.Rows.Add(3, "Suzanne Mathews", "France");
        dt.Rows.Add(4, "Robert Schidner", "Russia");
        dlContacts.DataSource = dt;
        dlContacts.DataBind();
    }
}

protected void ImageButton1_Click(object sender, ImageClickEventArgs e)
{
    foreach (DataListItem item in dlContacts.Items)
    {
        (item.FindControl("ImageButton1") as ImageButton).BorderStyle = BorderStyle.None;
    }
    ImageButton img = sender as ImageButton;
    img.BorderStyle = BorderStyle.Solid;
    img.BorderWidth = 2;
    img.BorderColor = System.Drawing.Color.Red;
}

Screenshot

Highlighted ImageButton